3 things imo.
As much as I despise original TBC and what it did to WoW as a whole, the reason imo, is not as much to do with the game as it is, but because the hype in the beginning or Classic Vanilla was huge and drew players of old back - even those that may not have had time and no longer play anymore. Then as they realized they couldn’t do it, they quit. If a lot of them found out they just didn’t have the time, why would they try TBC?
The second is that TBC Classic is grindier than Vanilla Classic. There are fewer shortcuts. This leaves a lot of people like they’re falling behind.
The third is raid sizes. in Vanilla you had to horde up 40 people. Now it’s 25. “Fat” is trimmed leaving those who didn’t put in the time in the beginning not being part of the core group. It’s probably demoralizing.
Edit: the raid size of 25 is so dumb. Why not make it a nice even number like 20.